This book's central personage suffers from a condition known as anhedonia, which is the removal of pleasure from one's experience of reality. Having been so disenfranchised from the majority culture in New York City, he feels detached from reality there, perhaps overwhelmed, or perhaps depressed. His mental health is not so bad though, because he finds meaning in his psychiatry work, and also because he takes long walks to clear his mind.
Those walks are the point of the book, no doubt. Notice that when he leaves his office, he enters the unknown, and he takes his point of view with him. So, he encounters his ideas in a new light, and just as his body ambulates, so also his mind ambulates through ideas, taking turns and making associations, until finally he arrives at some sort of emotional clarity. That clarity tells him to pursue his grandmother in Germany to meet her, but he fails, so the point of the clarity isn't to succeed in some task, but rather, the clarity itself is the point.
There is an ironic contrast between his role as psychiatrist in society and his private role as patient of experience and reality, just like everyone else. And notice, in his professional opinion, a walk is a sufficient exercise for the mind. Allowing one's self to freely move across a city (safely of course, hopefully), they can find a kind of center in the maelstrom of human experience. Instead of finding his family, the novel simple shows the drone of business as usual in the Big Apple, like static around him.
